1. Field of the Invention
The present invention relates to a chromaticity conversion device which can realize color reproduction compatible with a display device different in light source chromaticity and gradation characteristics, a chromaticity conversion method, a display device, a display method, a recording medium, and a program.
2. Description of the Related Art
Colorimetry information obtained from image data displayed on a color video camera, a color scanner, a projector or the like is often indicated as values of RGB. Here, R denotes red, G denotes green, and B denotes blue, and as reference colors (called original stimuli or basic stimuli), monochromatic light of 700 nm is used for red, monochromatic light of 546.1 nm is used for green, and monochromatic light of 435.8 nm is used for blue.
In modern chromatics, the basis of psychophysical expression of color is a color matching function (Color Matching Functions) modeled on spectral characteristics of a standard colorimetric observer (CIE 1931 Standard Colorimetric Observer). The color matching functions indirectly express the sensitivity of a human eye.
FIG. 1 shows the color matching functions using an RGB color coordinate system. As shown in FIG. 1, in the case where the RGB color coordinate system is used, minus values appear in tristimulus values. Besides, in the RGB color coordinate system, since luminance must be calculated using the tristimulus values of each of RGB, the comparison of the luminance has been difficult.
Then, an XYZ color coordinate system is widely used. Information of an object color received by the standard observer is numerically expressed by definite integrals expressed by expressions (1), (2) and (3). FIG. 2 shows color matching functions using the XYZ color coordinate system. As shown in FIG. 2, in the case where the XYZ color coordinate system is used, tristimulus values do not take a minus value.
[Numerical Formula 1]X=k∫visφ(λ)·{overscore (x)}(λ)dλ  (1)
[Numerical Formula 2]γ=k∫visφ(λ)·{overscore (y)}(λ)dλ  (2)
[Numerical Formula 3]Z=k∫visφ(λ)·{overscore (z)}(λ)dλ  (3)
Where, k is a constant and is expressed by expression (4). Besides, in the expressions (1) to (3), the integration is performed in a visible wavelength region (380 nm to 780 nm). Besides, φ(λ) is expressed by φ(λ)=R(λ)×P(λ) for a reflecting object, and is expressed by φ(λ)=T(λ)×P(λ) for a transparent object. Where, P(λ) is a spectral distribution of illumination light, R(λ) is spectral reflectivity of the reflecting object, and T(λ) is spectral transmissivity of the transparent object.
[Numerical Formula 4]
                    k        =                  100                                    ∫                              v                ⁢                                                                  ⁢                i                ⁢                                                                  ⁢                s                                      ⁢                                                            P                  ⁡                                      (                    λ                    )                                                  ·                                                      y                    _                                    ⁡                                      (                    λ                    )                                                              ⁢                              ⅆ                λ                                                                        (        4        )            
Although X, Y and Z basically denote intensities of color lights of red, green and blue, they denote colors (imaginary colors) which do not actually exist. X denotes a color close to red having no brightness, Z denotes a color close to blue having no brightness, and Y denotes a color close to green having brightness. That is, only the Y value has brightness.
An xy color coordinate system is a method for expressing a color (color is expressed by chromaticity coordinates x and y) by mapping into a two-dimensional space in accordance with the following expressions (5) and (6) using the tristimulus values XYZ of colors.x=X/(X+Y+Z)  (5)y=Y/(X+Y+Z)  (6)
In the case where z corresponding to Z of the tristimulus values XYZ of colors is considered, the following expression (7) is established.z=Z/(X+Y+Z)=1−x−y  (7)
As expressed in the following expressions (8) to (11), RGBW (W denotes white) can be expressed by using the tristimulus values XYZ.
[Numerical Formula 5]{overscore (R)}=Xr{overscore (X)}+Yr{overscore (Y)}+Zr{overscore (Z)}  (8)
[Numerical Formula 6]{overscore (G)}=Xg{overscore (X)}+Yg{overscore (Y)}+Zg{overscore (Z)}  (9)
[Numerical Formula 7]{overscore (B)}=Xb{overscore (X)}+Yb{overscore (Y)}+Zb{overscore (Z)}  (10)
[Numerical Formula 8]
                                                        W              =                                                R                  _                                +                                  G                  _                                +                                  B                  _                                                                                                        =                                                X                  ⁢                                                                          ⁢                  w                  ⁢                                                                          ⁢                                      X                    _                                                  +                                  Y                  ⁢                                                                          ⁢                  w                  ⁢                                                                          ⁢                                      Y                    _                                                  +                                  Z                  ⁢                                                                          ⁢                  w                  ⁢                                                                          ⁢                                      Z                    _                                                                                                          (        11        )            
Here, from the foregoing expressions (5) to (7), the following expressions (12) and (13) are established.X=Y(x/y)  (12)Z=Y(z/y)  (13)
Accordingly, from the expressions (8) to (13), the following expressions (14) to (16) can be obtained.
                                                                        X                ⁢                                                                  ⁢                w                            =                                                X                  ⁢                                                                          ⁢                  r                                +                                  X                  ⁢                                                                          ⁢                  g                                +                                  X                  ⁢                                                                          ⁢                  b                                                                                                        =                                                x                  ⁢                                                                          ⁢                                      r                    ⁡                                          (                                              Y                        ⁢                                                                                                  ⁢                                                  r                          /                          y                                                ⁢                                                                                                  ⁢                        r                                            )                                                                      +                                  x                  ⁢                                                                          ⁢                  g                  ⁢                                                                          ⁢                                      (                                          Y                      ⁢                                                                                          ⁢                                              g                        /                        y                                            ⁢                                                                                          ⁢                      g                                        )                                                  +                                  x                  ⁢                                                                          ⁢                                      b                    ⁡                                          (                                              Y                        ⁢                                                                                                  ⁢                                                  b                          /                          y                                                ⁢                                                                                                  ⁢                        b                                            )                                                                                                                              (        14        )                                          Y          ⁢                                          ⁢          w                =                              Y            ⁢                                                  ⁢            r                    +                      Y            ⁢                                                  ⁢            g                    +                      Y            ⁢                                                  ⁢            b                                              (        15        )                                                                                    Z                ⁢                                                                  ⁢                w                            =                            ⁢                                                Z                  ⁢                                                                          ⁢                  r                                +                                  Z                  ⁢                                                                          ⁢                  g                                +                                  Z                  ⁢                                                                          ⁢                  b                                                                                                        =                            ⁢                                                z                  ⁢                                                                          ⁢                  r                  ⁢                                                                          ⁢                                      (                                          Y                      ⁢                                                                                          ⁢                                              r                        /                        y                                            ⁢                                                                                          ⁢                      r                                        )                                                  +                                  z                  ⁢                                                                          ⁢                                      g                    ⁡                                          (                                              Y                        ⁢                                                                                                  ⁢                                                  g                          /                          y                                                ⁢                                                                                                  ⁢                        g                                            )                                                                      +                                  z                  ⁢                                                                          ⁢                                      b                    ⁡                                          (                                              Y                        ⁢                                                                                                  ⁢                                                  b                          /                          y                                                ⁢                                                                                                  ⁢                        b                                            )                                                                                                                                              =                            ⁢                                                                    (                                          1                      -                                              x                        ⁢                                                                                                  ⁢                        r                                            -                                              y                        ⁢                                                                                                  ⁢                        r                                                              )                                    ⁢                                      (                                          Y                      ⁢                                                                                          ⁢                                              r                        /                        y                                            ⁢                                                                                          ⁢                      r                                        )                                                  +                                                      (                                          1                      -                                              x                        ⁢                                                                                                  ⁢                        g                                            -                                              y                        ⁢                                                                                                  ⁢                        g                                                              )                                    ⁢                                      (                                          Y                      ⁢                                                                                          ⁢                                              g                        /                        y                                            ⁢                                                                                          ⁢                      g                                        )                                                  +                                                                                                       ⁢                                                (                                      1                    -                                          x                      ⁢                                                                                          ⁢                      b                                        -                                          y                      ⁢                                                                                          ⁢                      b                                                        )                                ⁢                                  (                                      Y                    ⁢                                                                                  ⁢                                          b                      /                      y                                        ⁢                                                                                  ⁢                    b                                    )                                                                                                        =                            ⁢                                                (                                                            Y                      ⁢                                                                                          ⁢                                              r                        /                        y                                            ⁢                                                                                          ⁢                      r                                        +                                          Y                      ⁢                                                                                          ⁢                                              g                        /                        y                                            ⁢                                                                                          ⁢                      g                                        +                                          Y                      ⁢                                                                                          ⁢                                              b                        /                        y                                            ⁢                                                                                          ⁢                      b                                                        )                                -                                                                                                       ⁢                                                {                                                            x                      ⁢                                                                                          ⁢                                              r                        ⁡                                                  (                                                      Y                            ⁢                                                                                                                  ⁢                                                          r                              /                              y                                                        ⁢                                                                                                                  ⁢                            r                                                    )                                                                                      +                                          x                      ⁢                                                                                          ⁢                                              g                        ⁡                                                  (                                                      Y                            ⁢                                                                                                                  ⁢                                                          g                              /                              y                                                        ⁢                                                                                                                  ⁢                            g                                                    )                                                                                      +                                          x                      ⁢                                                                                          ⁢                                              b                        ⁡                                                  (                                                      Y                            ⁢                                                                                                                  ⁢                                                          b                              /                              y                                                        ⁢                                                                                                                  ⁢                            b                                                    )                                                                                                      }                                -                                                                                                       ⁢                              (                                                      Y                    ⁢                                                                                  ⁢                    r                                    +                                      Y                    ⁢                                                                                  ⁢                    g                                    +                                      Y                    ⁢                                                                                  ⁢                    b                                                  )                                                                                        =                            ⁢                                                Y                  ⁢                                                                          ⁢                                      r                    /                    y                                    ⁢                                                                          ⁢                  r                                +                                  Y                  ⁢                                                                          ⁢                                      g                    /                    y                                    ⁢                                                                          ⁢                  g                                +                                  Y                  ⁢                                                                          ⁢                                      b                    /                    y                                    ⁢                                                                          ⁢                  b                                -                                  X                  ⁢                                                                          ⁢                  w                                -                                  Y                  ⁢                                                                          ⁢                  w                                                                                        (        16        )            
Besides, the following expressions (17) and (18) are established.
                    Xw        =                  xw          ⁡                      (                          Yw              /              yw                        )                                              (        17        )                                                                    Zw              =                                                (                                      1                    -                    xw                    -                    yw                                    )                                ⁢                                  (                                      Yw                    /                    yw                                    )                                                                                                        =                                                Yw                  /                  yw                                -                Xw                -                Yw                                                                        (        18        )            
Accordingly, expression (19) is obtained from the expression (14) and the expression (17), and expression (20) is obtained from the expression (16) and the expression (18).xw(Yw/yw)=xr(Yr/yr)+xg(Yg/yg)+xb(Yb/yb)  (19)Yw/yw=Yr/yr+Yg/yg+Yb/yb  (20)
Then, from the expressions (15), (19) and (20), Yr, Yg and Yb can be obtained as expressed by the following expressions (21) to (23)
[Numerical Formula 9]
                    Yr        =                  -                                                                                                                Yw                      ·                      y                                        ⁢                                                                                  ⁢                                          r                      (                                                                        xw                          ·                          yb                                                -                                                  xg                          ·                          yb                                                -                                                  xb                          ·                                                                                                                                                                                                            yw                      +                                              xg                        ·                        yw                                            +                                              xb                        ·                        yg                                            -                                              xw                        ·                        yg                                                              )                                                                                                                                            yw                    (                                                                  xg                        ·                        yb                                            -                                              xr                        ·                        yb                                            -                                              xb                        ·                        yg                                            +                                                                                                                                                                                      xr                        ·                        yg                                            +                                                                        xb                          ·                          y                                                ⁢                                                                                                  ⁢                        r                                            -                                                                        xg                          ·                          y                                                ⁢                                                                                                  ⁢                        r                                                              )                                                                                                          (        21        )            
[Numerical Formula 10]
                    Yg        =                  -                                                                                                                Yw                      ·                      y                                        ⁢                                                                                  ⁢                                          g                      (                                                                                                    -                            xw                                                    ·                          yb                                                +                                                  xr                          ·                          yb                                                +                                                  xb                          ·                                                                                                                                                                                                            yw                      -                                              xr                        ·                        yw                                            -                                                                        xb                          ·                          y                                                ⁢                                                                                                  ⁢                        r                                            +                                                                        xw                          ·                          y                                                ⁢                                                                                                  ⁢                        r                                                              )                                                                                                                                            yw                    (                                                                  xg                        ·                        yb                                            -                                              xr                        ·                        yb                                            -                                              xb                        ·                        yg                                            +                                                                                                                                                                                      xr                        ·                        yg                                            +                                                                        xb                          ·                          y                                                ⁢                                                                                                  ⁢                        r                                            -                                                                        xg                          ·                          y                                                ⁢                                                                                                  ⁢                        r                                                              )                                                                                                          (        22        )            
[Numerical Formula 11]
                    Yb        =                                                                              Yw                  ·                                      ybg                    (                                                                  xg                        ·                        yw                                            -                                              xr                        ·                        yw                                            -                                              xw                        ·                                                                                                                                                                                      yg                    +                                          xr                      ·                      yg                                        +                                          xw                      ·                      y                                        ⁢                                                                                  -                                                                  xg                        ·                        y                                            ⁢                                                                                          ⁢                      r                                                        )                                                                                                                          yw                  (                                                            xg                      ·                      yb                                        -                                          xr                      ·                      yb                                        -                                          xb                      ·                      yg                                        +                                                                                                                                                                  xr                      ·                      yg                                        +                                                                  xb                        ·                        y                                            ⁢                                                                                          ⁢                      r                                        -                                                                  xg                        ·                        y                                            ⁢                                                                                          ⁢                      r                                                        )                                                                                        (        23        )            
As typical color television systems, there are three systems of NTSC (National TV Standards Committee), PAL (Phase Alternating Line), and SECAM (Sequential Color And Memory). With respect to calorimetric parameters, constants peculiar to the respective systems are determined.
For example, in the NTSC system, since a transmitted signal is compressed at a transmission side, it has nonlinear gamma (γ) characteristics. Since the following expression (24) is established at the transmission side, the transmitted signal is expressed by expression (25).
[Numerical Formula 12]
                                          (                                                            R                                                                              G                                                                              B                                                      )                                n            ⁢                                                  ⁢            t            ⁢                                                  ⁢            s            ⁢                                                  ⁢            c                          =                                            (                              n                ⁢                                                                  ⁢                t                ⁢                                                                  ⁢                s                ⁢                                                                  ⁢                c                            )                                      -              1                                ⁢                                    (                                                                    X                                                                                        Y                                                                                        Z                                                              )                                      n              ⁢                                                          ⁢              t              ⁢                                                          ⁢              s              ⁢                                                          ⁢              c                                                          (        24        )            
[Numerical Formula 13]
                                          (                                                            r                                                                              g                                                                              b                                                      )                                n            ⁢                                                  ⁢            t            ⁢                                                  ⁢            s            ⁢                                                  ⁢            c                          =                                            (                                                                    R                                                                                        G                                                                                        B                                                              )                                      n              ⁢                                                          ⁢              t              ⁢                                                          ⁢              s              ⁢                                                          ⁢              c                                      1                              γ                ⁢                                                                  ⁢                n                ⁢                                                                  ⁢                t                ⁢                                                                  ⁢                s                ⁢                                                                  ⁢                c                                              =                                    (                                                                    (                    ntsc                    )                                                        -                    1                                                  ⁢                                                      (                                                                                            X                                                                                                                      Y                                                                                                                      Z                                                                                      )                                                        n                    ⁢                                                                                  ⁢                    t                    ⁢                                                                                  ⁢                    s                    ⁢                                                                                  ⁢                    c                                                              )                                      1                              γ                ⁢                                                                  ⁢                n                ⁢                                                                  ⁢                t                ⁢                                                                  ⁢                s                ⁢                                                                  ⁢                c                                                                        (        25        )            
Here, the matrix (ntsc) is a transformation matrix of three rows and three columns in the NTSC system.
Here, when respective elements of the transformation matrix of three rows and three columns are made {a11, a12, a13, a21, a22, a23, a31, a32, a33}, the respective elements are expressed by the following expressions (26) to (34), and XYZ can be expressed by the following expressions (35) to (37), respectively.a11=(xr*Yr)/yr*Yw)  (26)a12=(xg*Yg)/(yg*Yw)  (27)a13=(xb*Yb)/(yb*Yw)  (28)a21=Yr/Yw  (29)a22=Yg/Yw  (30)a23=Yb/Yw  (31)a31={(1−xr−yr)*Yr}/(yr*Yw)  (32)a32={(1−xg−yg)*Yg}/(yg*Yw)  (33)a33={(1−xb−yb)*Yb}/(yb*Yw)  (34)X=(a11*R+a12*G+a13*B)*Yw/255  (35)Y=(a21*R+a22*G+a23*B)*Yw/255  (36)Z=(a31*R+a32*G+a33*B)*Yw/255  (37)
Besides, when the respective elements of an inverse matrix of the matrix expressed by the expressions (26) to (34) are made {b11, b12, b13, b21, b22, b23, b31, b32, b33}, the respective elements are expressed by the following expressions (38) to (46)b11=(−a23*a32+a22*a33)/α  (38)b12=(a13*a32−a12*a33)/α  (39)b13=(−a13*a22+a12*a23)/α  (40)b21=(a23*a31−a21*a33)/α  (41)b22=(−a13*a31+a11*a33)/α  (42)b23=(a13*a21−a11*a23)/α  (43)b31=(−a22*a31+a21*a32)/α  (44)b32=(a12*a31−a11*a32)/α  (45)b33=(−a12*a21+a11*a22)/α  (46)
Where, α in the expressions (38) to (46) is a value expressed by the following expression (47).
                    α        =                                            -              a13                        *            a22            *            a31                    +                      a12            *            a23            *            a31                    +                      a13            *            a21            *            a32                    -                      a11            *            a23            *            a32                    -                      a12            *            a21            *            a33                    +                      a11            *            a22            *            a33                                              (        47        )            
FIG. 3A shows the relation between light source chromaticity points and names of variables, and FIG. 3B shows light source chromaticity points in the linear NTSC. In accordance with the names of the variables shown in FIG. 3A, the values of the light source chromaticity points in the linear NTSC shown in FIG. 3B are substituted for the expressions (26) to (34), so that the transformation matrix (ntsc) can be obtained.
[Numerical Formula 14]
                              (          ntsc          )                =                  (                                                    0.607                                            0.174                                            0.200                                                                    0.299                                            0.587                                            0.114                                                                    0.000                                            0.066                                            1.116                                              )                                    (        48        )            
It is very difficult to accurately confirm subtle color and brightness of image data, which has been processed by a predetermined system, such as the NTSC system described above, and has been transmitted, on a display at a receiving side. This is because the luminance of a fluorescent material of the display does not intensify in proportion to the intensity of an electron beam but reacts nonlinearly, and this is called nonlinearity of the display. Similarly, the human visual sense nonlinearly reacts to the fluctuation in luminance. In order to make this nonlinearity approach the human vision, a specific color map corresponding to a display to be used is prepared, and a color correction is carried out on the basis of the color map, which is called a gamma (γ) correction of the display.
There are gamma characteristics peculiar to each display. For example, also in a CRT (Cathode Ray Tube) display, the level of an input signal is not in proportion to (nonlinear) an amount of light emission of a fluorescent material. This is a phenomenon which is caused by the relation between a voltage applied to an electron gun of the CRT and an amount of emitted electrons. If the γ characteristics of the displays are not adjusted, the respective hues are changed, so that accurate color reproduction can not be carried out. Then, in the case where an image is displayed on the CRT display, in order to adjust the γ characteristics, the gamma correction corresponding to the CRT display is executed.
For example, as shown in FIG. 4, in a CRT nonlinear chromaticity conversion device 1 for displaying an NTSC signal on a not-shown CRT display, an inputted signal is processed by a signal processing portion 11 so that the tints are matched with the CRT display, and the gamma correction corresponding to the CRT display is carried out in a CRT γ characteristic operation portion 12. In the receiving side CRT display, the following expressions (49) and expression (50) are established.
[Numerical Formula 15]
                                          (                                                            X                                                                              Y                                                                              Z                                                      )                    crt                =                              (            crt            )                    ⁢                                          ⁢                                    (                                                                    R                                                                                        G                                                                                        B                                                              )                        crt                                              (        49        )            
[Numerical Formula 16]
                                          (                                                            R                                                                              G                                                                              B                                                      )                    crt                =                              (                                                            r                                                                              g                                                                              b                                                      )                    crt                      γ            ⁢                                                  ⁢            crt                                              (        50        )            
Here, the matrix (crt) is a transformation matrix of three rows and three columns in the CRT display.
In order to carry out accurate color reproduction of received signals, the following expression (51) must be satisfied.
[Numerical Formula 17]
                                          (                                                            X                                                                              Y                                                                              Z                                                      )                    crt                =                              (                                                            X                                                                              Y                                                                              Z                                                      )                    ntsc                                    (        51        )            
Then, since the following expression (52) is derived from the expressions (25), (49) and (50), in the case where signal processing is not carried out, the chromaticity of the CRT is exactly the same as that of regulations, and in the case of γcrt/γntsc=1 and (crt)=(ntsc), accurate color reproduction of a received signal is carried out.
[Numerical Formula 18]
                                                                                          (                                                                                    X                                                                                                            Y                                                                                                            Z                                                                              )                                crt                            =                                                (                  crt                  )                                ⁢                                                                  ⁢                                                      (                                                                                            R                                                                                                                      G                                                                                                                      B                                                                                      )                                    crt                                                                                                        =                                                (                  crt                  )                                ⁢                                                      (                                                                                            r                                                                                                                      g                                                                                                                      b                                                                                      )                                    crt                                      γ                    ⁢                                                                                  ⁢                    crt                                                                                                                          =                                                (                  crt                  )                                ⁢                                                                  ⁢                                                      (                                                                                            (                          ntsc                          )                                                                          -                          1                                                                    ⁢                                                                        (                                                                                                                    X                                                                                                                                                    Y                                                                                                                                                    Z                                                                                                              )                                                ntsc                                                              )                                                                              γ                      ⁢                                                                                          ⁢                      crt                                                              γ                      ⁢                                                                                          ⁢                      ntsc                                                                                                                              (        52        )            
However, practically, since the chromaticity of the NTSC is different from that of the CRT, (crt)=(ntsc) is not established. Besides, since there is an influence of signal processing for desirable picture quality formation at the receiving side, the γ characteristics at the transmission side and the reception side do not coincide with each other. Accordingly, for the color reproduction, a picture quality adjustment is carried out at the transmission side while the CRT display is seen, and a correction is carried out at the reception side, for example, RGB vector axes are shifted, so that the tints are matched with each other.
Next, a case in which the NTSC signal is displayed on, for example, an LCD (Liquid Crystal Display), not the CRT display, will be considered. The γ characteristics of the CRT display are different from the γ characteristics of the LCD. Since the processing relative to the CRT display is already established as a defect standard from the circumstances until now, a newcome display device must carry out the chromaticity conversion in accordance with the tints of the CRT.
FIG. 5 is a block diagram showing a structure of a conventional LCD nonlinear chromaticity conversion device 21 for displaying an NTSC signal on an LCD.
The inputted NTSC signal is subjected to a picture formation processing peculiar to the LCD, which is different from the processing carried out in the signal processing portion 11 explained by use of FIG. 4, in a signal processing portion 31. Then, in a color matching adjustment portion 32, the LCD light source chromaticity is made to approach the CRT light source chromaticity to the utmost, and the phases and levels of R-Y/G-Y vector axes are adjusted, so that a color divergence about which a user feels uneasy is adjusted in a natural picture. The color matching adjustment carried out here includes, for example, one in which vector axes for improving the color matching accuracy are made six axes in total, including magenta, cyan and yellow in addition to RGB and each of them is made adjustable, and one in which respective vector axes are made adjustable for every luminance level.
The signal subjected to the color matching is subjected to the inverse γ correction by 1/γ2 in an LCD inverse γ correction portion 33, and is subjected to the γ correction (γlcd) of the LCD in an LCD γ characteristic operation portion 34, and a generated picture signal of a luminance level (X, Y, Z) is outputted to a not-shown LCD and is displayed. Here, the relation among γ2 as they characteristics used for the inverse γ correction, γlcd as the γ characteristics used for the γ correction of the LCD, and γcrt as the γ characteristics of the CRT display as an object is expressed by the following expression (53).γlcd/γ2=γcrt  (53)
FIG. 6A shows the composite γ characteristics (that is, γ characteristics γcrt=2.5 of the CRT display) and the γ characteristics γlcd of the LCD. In order to adjust the difference of the gamma characteristics, the inverse γ correction is carried out in the LCD inverse γ correction portion 33. The inverse γ characteristics (1/γ2) in this case is shown in FIG. 6B.
However, in a nonlinear chromaticity conversion device, even if the γ characteristics are matched by using the foregoing method, if the light source chromaticity of each display does not coincide with each other, it is difficult to match colors seen by the use.
As measures against that, for example, there is a method in which not only the γ characteristics but also the light source chromaticity is also matched with the CRT display. However, in this case, exactly the same light source is required. Besides, there is a method in which after an input signal is returned linearly, chromaticity conversion is carried out, and then, the γ characteristics are matched by using a γ conversion circuit having the same characteristics as the CRT display. However, in this case, a color reproduction range becomes different. Further, although there is a method in which chromaticity conversion including a level as well is carried out, in that case, the processing becomes very complicated.
In any of the above measures, since the processing for matching the characteristics of a signal is carried out at the input side, it is necessary to carry out a processing for returning the characteristics of the signal according to the signal at the transmission side. Since the signal processing is carried out after the processing is ended, the signal conversion becomes very complicated.
The present invention has been made in view of such circumstances, and enables color reproduction compatible with a CRT at any signal levels by using simple linear chromaticity conversion.